In the field of liquid chromatography, it is necessary to precisely measure very small flow rates of liquids having different viscosities and under pressures commonly on the order of 5,000-6,000 psi. These liquids are the solvents which may be programmed sequentially through the liquid chromatographic column to elute various substances in a sample. The measured flow rates may be less than 10 microliters (.mu.l) per minute and even as low as 1-2 .mu.l per minute.
There have been various attempts to measure flow rates of a liquid by measuring displacement of a float or other object through a tube. This is also a feature of the present invention. However, the prior art systems were incapable of handling either the extremely low flow rates or the extremely high pressures of the apparatus of this invention. For example, in U.S. Pat. No. 3,662,598 of Spencer, there is disclosed apparatus for measuring the flow rate of an intravenous feeding system. The apparatus contemplates measuring the oscillatory frequency of a ball, a technique which requires a considerable clearance between the ball and the tube, thereby reducing the accuracy of the measurement. A somewhat similar technique employed on a much larger scale is disclosed in an article by A. T. J. Hayward entitled "Method of Calibrating Flowmeters with Liquids--A Comparative Survey" published in Measurement and Control, Volume 10, March 1977. The device described therein is entitled a "Pipe Prover" and is a device for calibrating large flowmeters. The article points out that one of its main disadvantages is that it occupies a considerable amount of floor space. The prior art devices required dynamic seals operating between system pressure and ambient pressure. This is acceptable only when the seal leak rate is small compared to the measured flow rate.
The unique characteristics of a liquid chromatography system, including extremely low flow rates and high pressures create special problems which prior art fluid flow measurement systems are poorly equipped to handle. For example, pump displacement cannot be relied upon as a satisfactory means for measuring flow because leaks from check valves and seals operating between system and atmospheric pressure become a significant percentage of the total flow. Also, the leakage within a pump varies with column back pressure, as experienced with composition gradients between liquids of different viscosities.
Accordingly, it is a primary object of the present invention to provide an improved flow metering system. Other objects are to provide such a system which is capable of precisely measuring and controlling low mass flow rates on the order of 10 .mu.l-3 ml per minute. Another object is to provide such a system which is capable of functioning under pressures up to approximately 6,000 psi. Another object is to provide such a system which is capable of functioning with fluids of substantially different viscosities. Another object is to provide a system which can be easily converted from one solvent to another without requiring flushing by large volumes of solvent. The manner in which the foregoing objects are achieved will be more apparent from the following description and appended claims.